In order to be able to operate LEDs directly on the mains, either a voltage conversion is necessary or the respective cascades of LEDs need to have a forward voltage which is in the region of the supply voltage. In this case, the last-mentioned case will be considered. Until now there have been two variants in this respect in addition to controllers switched at high frequency: firstly the LEDs are operated directly on the mains with a series resistor (so-called AC LEDs). Secondly, the LEDs are supplied with power via an in-phase controller, wherein the rectified voltage is smoothed in advance by a capacitor. In the first-mentioned variant, there are the disadvantages that the LEDs flicker at double the mains frequency or the LEDs are only on for less than half the time. Unfortunately, the second-mentioned variant is also disadvantageous since the absorption currents of the capacitors are very high in comparison with the operating current. Furthermore, the capacitors and rectifiers are overloaded during switchon since the switchon time at the mains is not defined. Finally, the power loss in the controller is undesirably high in the case of a design for the total mains tolerances.
It is known from the prior art to prevent flicker by using energy-storing components or by producing brightness modulations which are no longer visible to the eye.
Although the known electronic ballasts largely eliminate the problem of flicker, the electromagnetic interference produced in the process is undesirably high. If this is eliminated by EMC filters, additional costs are incurred. If this interference is not eliminated, the corresponding electronic ballasts are not suitable for certain applications.